Everything has its end. The longest and strongest El Niño event since 1950 is finally coming to an end, returning back to normal atmospheric and oceanic conditions (i.e. neither an El Niño nor a La Niña in place, or called ENSO-neutral state) this summer. While it is time to say goodbye to El Niño, we may need to welcome La Niña very soon. According to the latest observations and forecasts by a number of climate models around the world, there is an increasing chance that La Niña may start to develop in the latter part of the year. This has raised concerns from the public and media about the impact of La Niña on the weather of Hong Kong in the latter part of this year.
La Niña refers to the cooling of surface waters over the central and eastern equatorial Pacific Ocean which affects the atmospheric circulation worldwide and regional climate in many parts of the world. When La Niña is active during August to October, tropical cyclones in the vicinity of the Philippines are likely driven by an anomalous steering flow into the South China Sea and hence more tropical cyclones are likely to affect Hong Kong as compared with the ENSO-neutral state. According to past statistics, there could still be more than one tropical cyclone on average affecting Hong Kong in October under La Niña conditions (Figure 1). In other words, the tropical cyclone season may end later than in ENSO-neutral years or El Niño years.
Figure 1 Number of tropical cyclones coming within 500 km of Hong Kong in June-October during 1961-2015.
Regarding the impact of La Niña on the temperature in Hong Kong, although we may experience high temperatures when a tropical cyclone approaches Hong Kong, the effect will be short term and such impact to the mean temperature for the latter part of the year would be minimal. On the contrary, based on past statistics, the northeast monsoon affecting southern China is generally stronger in autumn (September-November) and winter (December-February) during La Niña, bringing lower temperatures to Hong Kong as compared with the ENSO-neutral state (Figure 2).
Figure 1 Autumn (September-November, left) and winter (December-February, right) temperatures of Hong Kong during 1950-2015.
Of course, whether the weather in the latter part of this year would follow the aforementioned pattern still depends on the development of La Niña and the influence of other factors.
For more information on El Niño and La Niña, please visit the Observatory webpage:
F C Sham and H W Tong
The rain season in Hong Kong this year has begun. Showers and thunderstorms occurred every now and then, sometimes affecting only isolated area and sometimes more intense and widespread. This to some extent affected the daily life of the public. Very often, forecasting heavy rain is a major challenge faced by the Observatory forecasters.
Nowadays, when forecasting the weather for the next few days, the meteorological community mainly relies on numerical weather prediction models run on computers, which simulate the evolution of atmosphere based on physical laws. Despite the advancement in science and technology, computer model predictions are still not perfectly accurate. It is especially challenging to predict weather phenomena such as thundery showers that change rapidly and affect relatively small areas. Generally speaking, the shorter the forecast lead time, the more accurate the computer model predictions will be. Therefore, for the weather forecast for a particular day, forecasters have to continuously assess the latest results of computer models as the subject day gets closer and closer, and consider whether or not to adjust the forecast.
We are usually able to predict environmental factors that favour the occurrence of heavy rain a few days in advance, such as whether the atmosphere will be unstable, and whether a trough of low pressure or an upper-air disturbance will come close to Hong Kong. Yet there is still a degree of randomness as to which hours of the day and at where the heavy rain will eventually occur, or whether the heavy rain will just narrowly miss Hong Kong. As such, for the forecast for the current day, forecasters still need to make reference to a vast amount of actual observation data, including those from automatic weather stations, weather radar and satellite to complement the forecast from the computer models; and if deemed necessary, immediately issue updates to provide a more accurate and detailed forecast to the public. This happens more often during spring and summer. Besides, another reason to update the forecast during the course of the day is that as the day progresses, the period covered by the forecast will shorten from 24 hours at the beginning to a few hours. The weather situation in the weather forecasts will naturally be different and therefore requires updates appropriately with time.
Heavy rain in spring and summer evolves rapidly. The rainfall distribution is also highly uneven. Coupled with the small geographical size of Hong Kong, any small deviations in the location and movement of rain areas can lead to drastically different local weather. As shown in Figure 1, there were two episodes of heavy rain in April 2016, which fell on our neighboring Shenzhen and the waters south of Hong Kong respectively, yet no heavy rain or just localized rain in Hong Kong. These speak for themselves on the difficulty in forecasting heavy rain.
Figure 1 Radar imageries showing heavy rain over Shenzhen and waters south of Hong Kong (areas in yellow and red) but hardly any over Hong Kong.
My advice to all of you before going out in the rain season is: not to judge the weather condition elsewhere simply based on how heavy the rain is at where you are. Instead, please keep track of the weather forecasts of the Observatory, check the location and development of rain areas using radar imageries, and pay attention to the latest 'Special Weather Tips' issued by the Observatory. In doing so, you can grasp the latest situation and get better prepared.
Finally, I would also like to take this opportunity to remind everyone that thunderstorms could bring not only heavy rain but also severe gust, squall line, hail and even tornado. Given the rain and typhoon season has begun, those who work in sectors including marine, logistics, transport and construction should take precautionary measures to avoid objects being blown down by strong winds in heavy rain. Members of the public should also pay attention to the relevant weather warnings and 'Special Weather Tips' issued by the Observatory to ensure their own safety.
 Thunderstorms and squalls
 Severe Weather in Hong Kong
 How is a tornado rated?
We may still recall, after the speculation in the social media on the intense cold surge affecting Hong Kong in late January 2016, rumours of another intense cold surge in March were doing the rounds in mid-February, attracting much public and media interest. Based on the information captured on the social media websites, the intense cold surge "forecast" appeared to hinge on two things: (1) most of the ensemble members of the forecasts by US National Centers for Environmental Prediction (NCEP) predicting the Arctic Oscillation Index (AOI) to become negative again (around -2 to -3), with one of the members forecasting AOI to fall below -4 on 2 March; and (2) AOI also falling below -4 about a week before the intense cold surge reaching southern China in late January 2016. Yet, it turned out that the AOI on 2 March was only about -0.6 and the lowest value in early March was around -2 (Figure 1). As for the cold surge predicted, temperatures did drop to 10oC at the Observatory on 10 and 11 March, but nothing too intense as the figure only ranked about 90th among the daily minimum temperatures for March.
Figure 1 Daily AOI (source: NOAA)
The lesson learnt was that it was never too wise to attach undue importance to the "face value" of long range forecasts produced by computer models and to jump to conclusions about the future weather conditions at a specific location such as Hong Kong. NCEP correctly forecast the trend of AOI changing from positive in mid-February to negative in early March. However, most of the ensemble forecast members provided a quantitative forecast that deviated rather significantly from the actual value of AOI in early March. Just like our regular reminder for users of the Observatory's 9-day forecast, uncertainty and forecast errors normally increased as we made prediction further into the future and as such, the forecasts in terms of such long-term trends should be interpreted objectively and used judiciously for maximum benefits.
Online discussion has also revealed some misunderstanding about the linkage between negative AOI and intense cold surge affecting Hong Kong (or other places in southern China). While negative AOI is a good precursor for the likely southward incursion of cold air from the north, to what extent any particular region would be affected depends on the actual trajectory of the cold air and other location-specific contributing factors involved. Typically, it takes around two to three days for the cold air to reach the south China coast, and the minimum temperature associated with a cold surge is normally recorded around two days after surge arrival. Based on daily AOI data (since 1950) from the US National Oceanic and Atmospheric Administration (NOAA), we have identified days in December - March with AOI at or below -4 and looked at the minimum temperatures at the Observatory in the ensuing seven days (denoted by Tm). As shown in the histogram in Figure 2, Tm was at or below 7oC (very cold weather) for only about 7% of the cases and was actually above 12oC (cold weather) for more than 40% of the cases. It can therefore be readily discerned that there is no general rule linking negative AOI with intense cold surge in Hong Kong.
Figure 2 Minimum temperature at the Observatory in the seven days following a day with AOI ≤ -4 (AOI data source: NOAA)
S M Lee